Hello agility_mom,
I have yet to hear of anyone who has had success using the Krylon spray product. I've tried to follow up with some who said they intended to use it, but I have yet to get a good report back.

So, a better bet, in my opinion, is this product. It can be rolled on in several coats, resulting in a thicker and more even coating. I have applied it to the roof panels on my own greenhouse, when we replaced them over two years ago. So far, the panels look good, but time will tell.
Top Secret TS-100 silicone epoxy coating

(Note that the application directions for this product also require the purchase and use of a proprietary thinner, sold by the same company.)

Another Harbor Freight greenhouse owner, rtyu, used the Top Secret product three years prior to posting in a thread last December, and at the time of that post, was still very happy with the results. You can read his application comments, close to the end of this thread:
HFGH Polycarbonate Panels Failing, Part 2

I wish I had more results to post about, but rtyu's experience, and mine, are the only I've come across so far.

Since your climate will be similar to mine, I think it might be a good investment of roughly $100, to better protect your panels. However, I have to stop short of sounding like this is any kind of a guarantee; unfortunately it will probably take more years, and more experiments by more owners, before we know if this is the best solution to the annoying Harbor Freight panel problem.

My original (untreated) Harbor Freight roof panels barely made it to three years, in my sunny climate, with 60% Aluminet shadecloth on top, year round. The wall panels have lasted over six years, but they are yellowed and have small pinholes in the outside layer. (Not a very pretty sight.) When I replace them, I'll coat the outside with the Top Secret epoxy product, and cross my fingers.

Folks in milder climates seem to have fewer complaints, but I agree your Arizona climate will likely be a challenge to the Harbor Freight panels. Hope this helps a bit!

Mudhouse, that helps a lot.

I am so glad that you answered this. I was going to leave an all year shade cloth structure over it with a 60 to 70% cloth and then have side covers in hot weather only. Since you covered your top all year and they still failed, this is good info to know.
The coating is very doable and would still be cheaper than most other greenhouses.
How does the coating make the panels look?

I thought it might have added a very slight (and I mean, barely noticeable) gold or yellowish tint to the panels. When they are new, they're almost a blue tint, they're so nice and clear. Very slight difference after the two coats of coating had dried, but really no one else would notice. I used a low nap paint roller to apply the coating.

I've had no issues with the coating peeling, bubbling, chipping, or doing anything unacceptable. I wasn't sure if this would be the case, so I was relieved. And after looking at my older untreated (original) panels, which are now yellow, brittle, and filled with little holes, I am happy to exchange a tiny visual change for the prevention of the ugliness (and functional weakness) I have now.

I tried 40% Aluminet at first, on my roof; silly me, I thought anything more would screen out too much sun from my cacti collection. But I soon learned by watching my plants that they needed more protection. They grow very nicely under the 60%, and most things I grow are the kind of plants that want high light levels, or they become etiolated and misshapen. No doubt 60% would be too much for some parts of the country, but here in southern NM it has worked well.

You'll find that you'll have varying zones of shade within your greenhouse, even after the roof is entirely covered. I put my sun lovers on the south and west walls, and put my shade lovers under the benches. The center of the greenhouse, on top of the benches, is middle range light. The really wimpy shade lovers get to live under the benches on the north wall, the shadiest side of all. It's surprising how many "light zones" I have, even in a little 10x12 greenhouse.

I think it's a great idea to buy an inexpensive GH like the Harbor Freight, before building a main greenhouse. I have learned SO much from having this humble little structure, and I would have been lost before, trying to know what to build, or what I'd like to have.

Hope you have fun.